1. Field of the Invention
The present invention relates to a structure of a rotor for an outer rotor type brushless motor (BLDC), and in particular to a structure of a rotor for an outer rotor type brushless motor (BLDC) which can improve structural stiffness, restrict noise generation by reducing vibration during the rotation, efficiently cool a heat generated in the motor, decrease a fabrication cost and improve durability.
2. Description of the Background Art
As illustrated in FIG. 1, a general outer rotor type brushless motor (BLDC) includes: a stator 100 where a coil is wound round a magnetic core 10; a resin frame 220 in a predetermined shape fabricated with a resin by using a die; a rotor 200 positioned outside the stator 100 in order to be alternately rotated in the right and left directions; and a sensor unit 300 connected to the stator 100, detecting a position of a permanent magnet 210 of the rotated rotor 200. and sequentially transmitting a current to the stator 100.
A driving shaft 400 is inserted into a center portion of the rotor 200.
The structure of the rotor 200 will now be described in more detail.
As depicted in FIGS. 2a and 2b, in the conventional outer rotor type brushless motor, the resin frame 220 forming an outer shape of the rotor 200 is formed having a predetermined height, a permanent magnet supporting unit 222 connected with the permanent magnet 210 being vertically curved and extended in an upward direction, and curved toward the center portion, at the outer circumferential portion to a disc-shaped base unit 221.
A ring-shaped deposition groove 223 having a predetermined height and width is formed at the inner wail of the permanent magnet supporting unit 222. A ring-shaped back yoke 230 having a predetermined width is inserted into the deposition groove 223. The plurality of permanent magnets 210 are stacked and adhered to the inner side portion of the back yoke 230 at predetermined intervals in a circumference direction.
The back yoke 230 is fabricated by rolling a thin steel plate. and serves to form a magnetic circuit of the permanent magnet 210. The back yoke 230 and the permanent magnet 210 are formed in a single body by a thermoplastic resin.
On the other hand, a boss unit 224 having a predetermined height is formed at the center portion of the base unit 221. A through hole 224a is formed at the center portion of the boss unit 224. A serration unit 225 having a plurality of triangle-shaped teeth is formed at the inner circumferential surface of the through hole 224a.
A shaft serration unit 401 formed at the outer circumferential surface of the driving shaft 400 is inserted into the serration unit 225 of the resin frame 220, and thus the resin frame 220 and the driving shaft 400 are combined. A spacer 410 is inserted into the lower portion of the shaft serration unit 401 inserted into the serration unit 225 of the resin frame 220. A nut 420 is fastened to a lower portion of the spacer 410, namely an end portion of the driving shaft 400.
On the other hand, a radiation fan blade 226 and a radiation hole 227 are provided on the bottom surface of the base unit 221 in order to cool a heat which is always generated during the rotation of the rotor 200 by means of an external air inflow.
As shown in FIG. 2b, a plurality of radiation blades 226 are formed in the base unit 221 in a radial shape centering around the boss unit 224. The plurality of radiation blades 226 have a predetermined thickness and width, and are formed in a vertical direction from the boss unit 224 to the permanent magnetic supporting unit 222.
In addition, a plurality of radiation holes 227 are formed in the base unit 221 at predetermined intervals in a circumferential direction. The plurality of radiation holes 227 are positioned to form a concentric circle, and cross the radiation blades 226.
In the above-described rotor 200, the permanent magnets 210 are positioned having a predetermined space from the stator 100. The driving shaft 400 connected to the resin frame 220 is fixedly connected to other constitutional elements.
In the conventional outer rotor type brushless motor, when a current sequentially flows to the coil 20 wound round the stator 100, the rotor 200 is rotated according to interaction between the current flowing in the coil 20 and the permanent magnet 210. The rotation force of the rotor 200 is transmitted to other constitutional elements through the driving shaft 400.
For example, in case the outer rotor type brushless motor adapts to a washing machine, the stator 100 is deposited in an outer casing including an inner casing, the driving shaft 400 is connected to the inner casing of the washing machine, and thus the driving force of the rotor can be transmitted to the devices such as the washing machine through the driving shaft 400.
During the rotation of the rotor 200, the air flows into the motor by the radiation fan blades 226 and the radiation holes 227, thereby cooling the heat generated in the motor.
However, while rotated by the interaction force with the current applied to the winding coil of the stator, as depicted in FIGS. 3a and 3b, the rotor for the conventional outer rotor type brushless motor is vibrated in a shaft direction and a radius direction.
The vibration is generated because the resin frame connected with the permanent magnet consists of the resin, and thus stiffness of the material is weak (approximately 15% of the steel plate). Especially, the vibration of the resin frame resulting from the vibration in the radius direction increases noise.
Moreover, since the frame consists of the resin, the serration unit of the frame connected to the driving shaft transmitting the driving force generated from the rotor is easily abraded under the operational conditions of high temperature, high torque and impact load, and thus a life span thereof is reduced.
In addition, the radiation fan blades for cooling the inside of the motor with the external air are formed in a vertical direction. Therefore, when the rotor is rotated in one direction, an amount of the air which flows into the motor and is discharged therefrom is increased. As the thermal conductivity of the resin is low, radiation is not efficient.
Furthermore, the frame consisting of the resin is very weak to a fatigue destruction resulting from a repeated stress generated by alternation of the washing machine. Accordingly, the radiation hole must be formed small. However, the small radiation holes cannot sufficiently perform a cooling operation. As a result, when the cooling operation is ill-performed, a resistance of the coil is increased, motor efficiency is reduced, a temperature of the coil is more increased, and thus the coil may be easily damaged. Consequently, an expensive coil of high quality must be used.
The resin frame consists of the resin, and thus a price thereof is relatively high. Also, it is necessary to separately fabricate and connect the back yoke in order to form the magnetic circuit, which results in increased production and assembly costs.